Quickslots are divided into Active Ability Quickslots and Consumable Quickslots.
You can toggle between them in combat by pressing.
Items and abilities assigned to Quickslots can be used by pressing,,,.
Consumable Quickslots have a maximum number of items that can be used from a single Quickslot within one combat encounter. This number can be increased by learning the Medicus I perk.
Consumable Quickslots have Night and Day loadouts. Those can be set up in the Inventory panel.
Active Ability Quickslots have Night and Day loadouts as well. Those can be set up in the Active Abilities panel.